Forschung

1998

Diendorfer G., W. Schulz:
Lightning Characteristics Based on Data from the Austrian Lightning Locating System

IEEE-EMC Transactions, Vol. 40, Number 4, 1998

In this paper we compare various lightning characteristics measured by the Austrian lightning locating system, ALDIS, with those found in the literature. The latter are typically based on measurements of lightning electric fields. We show that lightning peak electric fields due to subsequent strokes measured by the ALDIS are similar to those in the literature. However, the ALDIS data do not show the usual ratio of about 2:1 between the median values of the field peaks for first and subsequent strokes. Although the flash detection efficiency of the ALDIS system in the area of investigation is estimated to be higher than 90 %, one of the best for such systems all over the world, the observed percentage of single-stroke flashes and the average number of strokes per flash seem to suggest that the stroke detection efficiency is appreciably less than 90 %.The ALDIS data indicate that larger strokes are preceded by longer interstroke intervals. The mean flash duration of 175 ms measured by ALDIS is similar to the typical flash duration found in the literature. Strokes with larger field peaks tend to have higher average field rates of rise.

Since many lightning parameters show a large scatter for different thunderstorm days, long-term data from lightning locating systems are more representative of average lightning compared to data derived from electric field measurements typically performed during a few thunderstorms.

In this paper we compare various lightning characteristics measured by the Austrian and the French lightning location system. Both systems are lightning location networks using an APA283 position analyzer and IMPACT sensors. The two lightning location systems differed during the period of investigation mainly by the mean baseline between the direction finder (DF) and by the configuration parameter "number of sent strokes" of the DF. This parameter is set at the sensor itself (LINK_FORMAT: strokes) and limits the number of strokes per flash sent by the DF to the position analyzer.

We show in this paper how the difference in the mean baseline and the configuration parameters influence the lightning peak current distributions, the interstroke intervals, the number of strokes per flash, and the average flash duration. We also compare the lightning parameters reported by these two lightning location systems to lightning parameters available in literature.

Diendorfer G., W. Schulz, M. Mair:
Effects of Field Propagation on the Peak Current Estimates

Accuracy of peak currents inferred from peak magnetic fields is dependent on the attenuation of peak fields due to finite ground conductivity. In our presentation we will show significant differences in signal strength reduction caused by propagation paths of dissimilar conductivity over the Austrian territory. Comparing time correlated data from direct lightning peak current measurements from an instrumented tower at a minimum distance of about 70 km to a DF in the Austrian mountains reveals differences in range normalized signal strength of more than 50%. Attenuation models that are only distance dependent are not applicable to correct this effects observed in our location system. In this case, a distance and angle dependent attenuation model, accounting for the actual propagation path is required.

Based on the sensor data from the LLS we have determined for a grid of 10 km x 10 km an estimate for the ratio signal strength measured by the sensor to the mean of the range normalized signal strength of all sensors reporting a stroke. Contour plots of this ratio clearly show regions of significant differences in field propagation effects.

We also compare parameters from lightning fields (rise time and pulse width) measured with an identical experimental setup in Florida and in Austria. Differences found in this comparison are assumed to be mainly caused by the differences in ground conductivity.

With lightning location systems an estimation of the peak current of the lightning discharge can be performed. The accuracy of this estimation is influenced by the finite ground conductivity of the propagation path of the electromagnetic field. In this paper we present an empirical method for taking into account the attenuation due to real ground conditions on the estimation of the lightning peak current. The resulting peak current distribution is compared to the results when the traditional method is used.

Recent investigations of site errors have shown that site errors are not only caused by nearby visible objects (e.g. buildings, overhead lines) of a direction finder site. Site errors are also related to the direction and the grounding conditions of the buried power and communication cable connected to the direction finder.

In this paper we present measurements of lightning electromagnetic fields and corresponding induced currents in the shield of the communication cable. We show also that the amplitude of the induced current varies with angle between the direction of the cable and the direction to the lightning location. The induced currents in the cable shield due to the lightning electromagnetic field are causing a significant portion of the observed site error.

In 1997 and spring 1998 at the Peissenberg tower 8 flashes with 44 strokes (12 &alpha; and 32 &beta; type impulse currents) having peak currents greater than 4 kA have been measured. For the definition of &alpha; and &beta; type impulse currents see Zundl et. al 1996. We have correlated reports from the Austrian Lightning Location System (ALDIS) for 35 of those 44 strokes. Measured peak currents of less than 4 kA were not included in this analysis, because this discharges are unlikely to be located by ALDIS due to the distance of the tower to the location network.

We found correlated lightning locations for 75% (9 out of 12) of the &alpha;-type impulse currents and for 81% (26 out of 32) for the &beta;-type impulse currents. This indicates a better detection efficiency of the system for &beta;-components than for &alpha;-components. &beta;-components are assumed to be more similar to subsequent strokes.

The location system infers peak currents from measured peak magnetic fields using a calibration function Ip= f(Bp) assuming a 1/R distance dependency for the propagation of the electromagnetic fields. This is exact only for perfectly conducting ground.

Several spots of significant high flash density have been discovered in Austria. Most of these spots are associated with different types of radio towers on top of mountains.

We have analyzed flashes that have been detected from a lightning location system during a period of two years at a distance of less than 1 km from radio towers. Most of these flashes are assumed to be direct strikes to the tower. In average their peak amplitudes and their number of subsequent strokes are significantly higher compared to flashes to ground in the vicinity of the tower.